//declare property

Button buttonOpenFile;

    ListView listView;

    ArrayList<String> dataList;

//write in onCreate method    

 buttonOpenFile=findViewById(R.id.buttonOpenFile);

        listView = findViewById(R.id.listData);

        dataList=new ArrayList<>();

        buttonOpenFile.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);

                startActivityForResult(intent, 42);

            }

        });

 //write this function outside of onCreate method

 public void onActivityResult(int requestCode, int resultCode, Intent resultData) {

        super.onActivityResult(requestCode, resultCode, resultData);

        if (resultCode == RESULT_OK) {

            Uri treeUri = resultData.getData();

            DocumentFile pickedDir = DocumentFile.fromTreeUri(this, treeUri);

​

            // List all existing files inside picked directory

            for (DocumentFile file : pickedDir.listFiles()) {

                Log.d("TAG", "Found file " + file.getName() + " with size " + file.length());

                dataList.add(file.getName());

            }

​

            ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,

                    R.layout.list_item, R.id.label,dataList);

            listView.setAdapter(adapter);

​

        }

    }